This invention relates generally to the production of cathode ray tubes, and particularly to an apparatus for centering a deflection yoke in a support ring for a test set.
The visual display on the screen of a cathode ray tube (CRT) is generated by scanning electron beams across the screen of the cathode ray tube. During the scanning, the electron beams are modulated with the video information required to generate the desired display. In a CRT for a color television receiver, three beams are simultaneously scanned across the screen. The electron beams are scanned horizontally across the screen from one side to the other. After a complete horizontal line is scanned, the electron beams are returned to the initial side so that all horizontal lines are scanned in the same direction. Prior to scanning the succeeding line, the electron beams are stepped vertically one line width. The horizontal scanning and vertical stepping are repeated until the full screen has been scanned by the electron beams and one frame of the display been generated.
The horizontal scanning and vertical stepping of the electron beams are affected by a magnetic deflection device, commonly called a yoke. The yoke includes both horizontal and vertical deflection coils which are biased with the appropriate voltages to cause the scanning and stepping. The horizontal deflection coils are actuated with a varying signal, such as a triangular waveform to cause the beams to scan horizontally across the tube. The vertical deflection coils are provided with vertical deflection signals which cause the beams to impact the screen at selected vertical positions in synchronization with the initiation of the horizontal scanning. It is important that the horizontal and vertical electron beam motions be parallel to the horizontal and vertical axes of the screen. The orientation of the yoke on the CRT is therefore important because the positioning of the deflection coils in the yoke determines the direction of the beam motions. One of the final steps in the manufacture of a kinescope is the testing of the tube to verify that all operating parameters are within the required specifications. This test is conducted prior to permanently placing a yoke on the kinescope. Typically, the test is done in a test set which includes a yoke and which has provisions for applying the horizontal scanning voltage and the vertical stepping voltage to the respective deflection coils of the yoke. The yoke in the test set must be of the type required for the type of tube to be tested. Accordingly, each time a different type of tube is tested, the yoke in the test set must be changed. In the past, a different yoke mounting arrangement was required for each type of tube. This requirement resulted in substantial expense in designing yoke mounting arrangements. For these reasons, there is a need for an apparatus for accurately and rapidly centering yokes for all tube types in a test set used to test cathode ray tubes. The present invention fulfills this need.